1. Field of the Invention
The present invention relates to a circuit to check overerasing of repair fuse cells, and more particularly to a method and a circuit to check overerasing of repair fuse cell for checking the overerasing state of fuse cells by sensing the current flowing in the fuse cells at the state to apply 0V to gates of all fuse cell and a uniform voltage to the drain, entering the sensing output to the logical gate and reading the output of the logical gate through an input/output pad.
2. Description of the Prior Art
Programming repair fuse cells allows repairing defective cells. Then the initial fuse cells should have been erased, and the threshold voltage of the cells should also be adjusted low with enough margins. Where the initial fuse cells were programmed, the normal cells will be repaired since the fuse cells were programmed although the normal cells did not need repairing. Thus undesirable repair occurs so that redundant cells for repairing are reduced.
On the other hand, the threshold voltage of the initial fuse cells should be low enough to remove the problem to cause inferiority by access to the redundant cells for repairing to act differently from the main array cells by programming by charge gain of fuse cells that may be generated during testing under some adverse conditions. Accordingly, at the early stage of testing, the fuse cells for repairing should be erased. and the threshold voltage should also be adjusted low. For the process, a circuit to check whether fuse cells are erased and, at the same time, to adjust the threshold voltage of the cells to be low enough is needed.
Accordingly, it is an object of the present invention to provide a circuit to check overerasing of repair fuse cells which can check whether the repair fuse cells are erased, and can adjust the threshold voltage of cells to be low enough at the same time.
The present invention to achieve the above objects comprises: a first means to select all repair fuse cells using a test mode and to sense the repair fuse cell state respectively in accordance with the applied read-out voltage; a logical means to generate decision signals to decide the overerasing state of said repair fuse cells by logically combining the sensed signals from each repair fuse cells; and a second means to read output signals of said logical means.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.